A couple of comments:
The general notion of spacing tribanders, or any antenna a "generic" 1/2
wavelength apart is not a very good idea. The vertical spacing depends upon the
gain (aperature) of the antennas to be used in the stack, as well as the
results, such as more forward gain, higher/less F/B, take-off angle(s), etc.
This is an excellent use of antenna modeling software.
Stacking single-boom multiband Yagis will usually result in not being able to
optimize all three bands for forward gain and F/B. This is, of course, simply
because the fixed spacing will not be able to favor all 3 bands. A two stack,
however, is well worth the effort. A trio is even better!
The one caveat to utilizing antenna modeling software for stacks is knowing the
actual forward gain of the antenna(s). If the gain is truly known, then the
models will be accurate; otherwise, the results will be speculative. Please
this is not intended to open the subject of trapped tribander gain. It is to
give a caution to be realistic in the evaluation process, or a lot of energy
will be expended and expectations probably will not be met.
The higher the forward gain, the greater the vertical (or horizontal) spacing.
As an extreme example, our highest gain Yagi antenna has 21dBi (at 5.8GHz). To
arrange a pair of these antennas in a vertical stack, the vertical separation
set at 3.6 wavelengths. A 3dB lower gain antenna (18dBi) is placed at 3.3
wavelengths. A simple chart follows:
NOTE---> all free space models, only looking for increased forward gain;
and anyone can do this!
Gain #Ele / Boom Length Vertical Spacing
6.6dBi (4.45dBd) 2 / 0.15 (10.5' on 20m) 0.4-0.6 wavelengths
7.6dBi (5.45dBd) 3 / 0.25 (17.5' on 20m) 0.5-0.7
8.8dBi (6.65dBd) 4 / 0.43 (30.1' on 20m) 0.7-0.9
9.9dBi (7.75dBd) 5 / 0.60 (42.0' on 20m) 0.9-1.1
10.5dBi (8.35dBd) 6 / 0.75 (52.5' on 20m) 1.0-1.1
11.6dBi (9.45dBd) 8 / 1.10 (77.0' on 20m) 1.3-1.4
Hope this sheds some light on the subject. As mentioned, this is an excellent
use for the computer model. My personal process for HF is to work in free space
to get an idea, then convert and do everything over real ground. Using Dean
Straw's (N6BV at ARRL) terrain analysis has been quite useful. It also promoted
a lot of thought on incoming and useful wave angles!
73, Tom, N6BT
Force 12 Antennas and Systems
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